An experimental study into the ultimate capacity of an ‘impression’ pile in clay

The ultimate capacity of a novel type of piled foundation called an ‘impression’ pile has been investigated using centrifuge modelling techniques. The name derives from the small discrete impressions created in the side walls of a bored cast in situ pile to increase the soil/pile friction such that the impressions form nodules on the shaft of the concreted pile. The technology is suitable for bored piles in overconsolidated clay, such as London Clay. The experiments explored the influence of geometrical parameters such as the vertical spacing of the impressions, their number at each level and their shape. The data show a consistent increase in pile capacity of 40% when the impressions extend over 85% of the pile length. The ultimate capacity of the pile is primarily affected by the length of the pile which is impressed, the number of nodules at a given cross-section and the spacing of the nodules in the vertical direction, as long as this is greater than a threshold value. According to the experimental evidence, a block failure occurs for a spacing lower than this threshold value. Plastic failure mechanisms for the impression pile were established. These were used successfully to calculate the ultimate capacity of the impression piles tested with an error of less than 10%.